1. Field of the Invention
The present invention relates to a traction control system for a vehicle capable of regulating driven wheel speeds by varying braking forces for driven wheels in accordance with excessive slipping of the driven wheels.
2. Description of the Related Art
Such a type of traction control system is conventionally known, for example, from Japanese Patent Application Laid-Open No. 3-109158. The traction control is basically utilized to converge the driven wheel speed into an optimal target wheel speed for preventing a reduction in accelerating efficiency due to excessive slipping of the driven wheels and a reduction in stability of the behavior of the vehicle body. Namely, the driven wheel speed is controlled by decreasing the engine output and by braking the driven wheels according to a difference between the target wheel speed and the driven wheel speed.
In the known traction control system, however, when the engine output is decreased by a driver's will, it cannot be definitely discriminated whether the reduction in slip rate of the driven wheels is caused by the traction control or by the driver's operation. Notwithstanding that the need for inhibiting the excessive slipping of the driven wheels has been eliminated, inappropriate traction control may be carried out, thereby providing an uncomfortable decelerating feeling to the driver.
For example, in a system designed to inhibit the excessive slipping of the driven wheels by controlling the braking pressure of a driven wheel brake, as shown in FIG. 5A, a target wheel speed is determined in consideration of a constant slip rate relative to the vehicle speed presumed based on at least the follower wheel speed, and a threshold value is determined slightly larger than such wheel speed. The braking pressure is increased as shown in FIG. 5C from the time point when the driven wheel speed exceeds the threshold value to start the traction control. However, when a throttle in an engine is brought to a fully-closed state, as shown in FIG. 5B, by the driver's will during carrying-out of the traction control, it cannot be definitely discriminated whether the reduction in slip rate due to a reduction in driven wheel speed is caused by the traction control or by the driver's operation, in spite of the fact that the reduction in driven wheel speed due to the reduction in engine output has been caused by the fully-closed value of the throttle opening degree. Therefore, the traction control based on the braking pressure is continued, whereby the braking pressure is gradually reduced after the driven wheel speed becomes less than the threshold value. For this reason, the driven wheel speed is excessively reduced relative to the follower wheel speed and hence, the deceleration of the vehicle may be increased more than necessary to provide an uncomfortable feeling to the driver.
The present invention has been accomplished with such circumstance in view, and it is an object of the present invention to provide a traction control system for a vehicle, wherein a deceleration greater than necessary is prevented from being produced, whenever a driver has acted so as to reduce the driven wheel speed during carrying-out of the traction control, thereby avoiding the uncomfortable feeling provided to the driver.